When there are electrostatic discharges (ESD), high voltages occur, for example between the terminals of an electric circuit. This can lead to high currents through the circuit. Particularly with integrated circuits, the circuit could be destroyed by this.
To protect a circuit against electrostatic discharges it is possible to provide circuit arrangements that can drain off a current if there is an electrostatic discharge, thus protecting the electric circuit from damage if a high voltage occurs.
In such circuit arrangements it is possible to use various types of diverting elements, which are in each case connected between the terminals of the circuit or a component that is to be protected. The various types differ, among other things, in their voltage-current characteristic curves, and accordingly can have different protective behaviors.
FIG. 14 shows an example of a voltage-current diagram with the voltage-current characteristic curves of different diverting elements, which here are represented as so-called transmission line pulsing (TLP) curves. For example, a first type of diverting element has the characteristic curve SFA, which has the form of a diode characteristic with breakdown voltage VB. The characteristic curve SFA does not have a voltage snapback. In contrast, a second example of a diverting element is determined by the characteristic curve SA, which has snapback from the breakdown voltage VB to a snapback voltage VR.
Protection of the circuit or component may additionally be necessary in various operating states. For example, a component in uninstalled state should be protected, for example against a discharge due to an electrostatic charge transferred to the component by contact and/or static electricity due to friction. Such protection in which a component in uninstalled state is protected can also be called chip-level protection. Moreover, it may be necessary to protect a component from overvoltages that occur at terminals of the component when it is installed in a circuit and/or when it is operated together with a circuit. Such overvoltages can occurs, for example, due to undesired transient processes, for instance at power supply lines or because of electrostatic discharges. Protection of a component in installed state or during the operation of the component can also be called system-level protection.
Accordingly, protective elements or protective structures for protection against electrostatic discharges can basically be better suited for one of several possible protective applications. Referring to the diagram in FIG. 14, for example, with a protective element having a characteristic curve corresponding to curve SA, the protective element, when operated at a supply voltage or a battery voltage VBAT, may become damaged if there is an overvoltage that is to be drained off. If, for example, a voltage between the terminals to be protected is higher than the breakdown voltage VB, the protective element is triggered, which initially causes a snapback to the snapback voltage VR. If, as in this example, the battery voltage is higher than the snapback voltage VR, the protective element is, however, no longer in a nonconducting state, but rather there will be a current increase up to a value that corresponds to the battery voltage VBAT. As a rule, this current will be higher than the permissible continuous current of the protective element, which in the end can lead to damage to the protective element.
Such a damaged protective element may effect a permanently conducting connection between the terminals to be protected. As a consequence, the device to be protected or the circuit to be protected possibly can no longer be used.
If protective elements with a snapback-free curve corresponding to curve SFA in FIG. 14 are used, as a rule, a considerably larger area is required on a semiconductor chip than in the case of a protective element with a curve corresponding to curve SA. This leads to higher cost and/or a larger circuit in the production of a component with a corresponding protective element. As a rule, this also involves higher costs.